1. Field of The Invention
The present invention relates to a legged walking robot and a system for controlling the same. More particularly, it relates to a system for controlling a drive joint of a legged walking robot which provides smooth posture control by eliminating interference or disturbance between the frictional force of the ground acting on the legs of the robot and the driving force of the leg joints. Furthermore, it relates to various designs for robots including a walking robot in which, for example, the axes of the drive joints are arranged at right-angles to one another thereby enabling positional control to be effected using Cartesian coordinates.
2. Description of The Prior Art
In recent years there has been extensive development of multi-jointed industrial robots, much like the one described in Japanese Laid-open Patent Publication No. 58(1983)-51090, or such as the biped walking robots described in Japanese Laid-open Patent Publication 62(1987)-97005 and 62(1987)-97006. In the case of multi-joint legged mobile robots such as, for example, the above mentioned two-legged mobile robot, controlling the posture by driving the joints with both legs in contact with the ground can cause interference or disturbance between the horizontal frictional force of the ground acting on the robot, and the sum of the horizontal driving power outputs of the robot's joints, producing a vibration or juddering. This phenomenon is caused by the closed link the two legs form with the ground, and because of the fact that positional control alone cannot absorb the degree of error arising in the course of the control.
With reference to FIG. 1, for example, when the robot has both legs in contact with the ground and the drive joints are driven to alter the posture, the cumulative positional error (degrees, in this case) in the output of the joints is manifested at the points of contact between each of the two legs and the ground, causing them to scrape along the ground. However, owing to the weight of the robot itself acting on the legs, the resistance of the frictional force acting in opposition to this movement subjects the links to an excessive force, while the intermittent sliding of the leg in contact with the ground causes judder. The overall result is that the robot is unable to move smoothly. In the case of stationary industrial robots, one way that is used to solve the problem consists of adjusting the control force if positional control operations give rise to interference. Hence, one reason which causes such a problem results from attempting to effect posture control in a mobile robot using only positional control. The drawbacks can be thus resolved by the use of force control in addition to position control.
The above problem also occurs when the robot is walking, as is demonstrated in FIG. 2. Assuming a robot in forward motion is being viewed from the front, the same problem will occur during the period when both legs are in contact with the ground. Similarly, if the robot is observed from the side as it is walking, it assumes a state in which the two legs are apart on the ground. The problem is complicated by the fact that the normal condition is for the surface reaction force to be towards the robot's center of gravity rather than vertically to the ground plane, the problem is the same with respect to the fact that error in the positional control of the joints produces an abnormal horizontal reaction force at the point of contact with the ground, making it necessary to resort to force (torque) control to reduce the horizontal reaction force to a normal level.